In paper coatings, kaolin clay is often a major component. Kaolin clay is traditionally manufactured by a series of steps which may include bleaching, floatation, particle size fractionation, magnet treatment, and dispersion. Like other pigments, such as titanium dioxide, calcium carbonate, and talc, it must be dispersed. However, unlike other pigments, it is traditionally dispersed in the final stages of processing from an acidic filter cake. Normally, the dispersion is carried out with the combination of soda ash and sodium polyacrylate. This produces an approximately 60% solids dispersed filter cake. Dried clay is then back-added to the dispersion to achieve a normal shipping solids content of about 70%.
Traditionally, in paper coatings, dispersed clay is used with other coating ingredients. A latex, such as styrene butadiene, with other functional monomers, such as itaconic acid, is often used. Although these lattices are usually employed with hydrocolloids which provide viscosity and water retention for the coating process, increased levels of carboxylation on the latex, or introduction of an associative thickener, may provide a sole binder system. Both sole binder lattices and hydrocolloids, such as carboxymethyl cellulose and hydroxyethyl cellulose, influence the wet coating structure. The result is that the flow characteristics are changed from the basic dispersed clay.
It is normal for the high shear viscosity to be increased. The solids of these coatings must be adjusted to provide runnability, required for a defect-free coating. With the trend towards increased blade coating speeds, a common approach toward formulation is to search for lower high shear viscosity. However, with lower high shear viscosity, achieved by either lowering solids content, or judicious selection of coating materials, a compromise in loss of holdout of the coating may occur. This may occur through pressure penetration in the roll applicator before the blade, pressure penetration into the paper at the blade, or penetration due to base stock decompression after the blade. The problem faced by the formulator of blade coatings thus continues to be one of coating design in terms of achieving a satisfactory balance between runnability and coating holdout.